CN113629754B

CN113629754B - Online switching test system and control method for hybrid direct current third station

Info

Publication number: CN113629754B
Application number: CN202110966598.XA
Authority: CN
Inventors: 王浩; 任军辉; 曹鹏; 王杰峰; 王彤辉; 宋志顺
Original assignee: China XD Electric Co Ltd; Xian XD Power Systems Co Ltd
Current assignee: China XD Electric Co Ltd; Xian XD Power Systems Co Ltd
Priority date: 2021-08-23
Filing date: 2021-08-23
Publication date: 2023-07-07
Anticipated expiration: 2041-08-23
Also published as: CN113629754A

Abstract

The invention discloses a hybrid direct current third station online switching test system and a control method, wherein the system comprises the following components: a transformer T1, a transformer T2, a transformer T3, a converter LCC, a converter VSC1 and a converter VSC2; the first end of the transformer T3 is connected with the alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC 1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected to the second end of the transformer T3, and the second end of the transformer T2 is connected to the ac side end of the converter VSC 2. The invention can realize the online input and the online exit of the third station under the hybrid direct current system, can be used for verifying the control and the protection strategy of the online input and the exit of the third station in the hybrid direct current transmission project, and can be used for verifying the feasibility of the online input and the exit technology of the third station in the actual project.

Description

Online switching test system and control method for hybrid direct current third station

Technical Field

The invention relates to the technical field of high-voltage direct-current transmission, in particular to a hybrid direct-current third station on-line switching test system and a control method.

Background

This section is intended to provide a background or context to the embodiments of the invention that are recited in the claims. The description herein is not admitted to be prior art by inclusion in this section.

Along with the application of the traditional LCC and VSC-based hybrid direct current transmission technology to the field of transmission, the method combines the superior control performance of flexible direct current transmission, solves the problem of commutation failure caused by weaker receiving-end alternating current system, has considerable transmission capacity of conventional direct current transmission, and has better technical economy. The multi-terminal hybrid direct current transmission system can be connected with a plurality of alternating current power grids with different delivery and absorption capacities, realizes multi-power supply and multi-drop point power reception, saves a transmission line corridor, and is a more flexible direct current transmission mode. With the development of the multi-terminal hybrid direct current technology, the multi-terminal hybrid direct current transmission requires that the third station can still have the online exit and input capability under the condition that other converter stations of the system are not locked. The third station on-line switching control system of the existing direct current transmission system comprises: three conventional LCC converter stations are not related to a hybrid dc test system comprising a converter LCC and a converter VSC and there is no complete test system loop.

Disclosure of Invention

The embodiment of the invention provides a hybrid direct current third station online switching test system, which is used for realizing online switching and switching of a third station under a hybrid direct current system, and comprises the following steps: a transformer T1, a transformer T2, a transformer T3, a converter LCC, a converter VSC1 and a converter VSC2; wherein, the positive and negative lines of the direct current side of the converter LCC are respectively provided with a direct current switch AK1 and a direct current switch AK2; a direct current switch BK1 and a direct current switch BK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 1; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC 1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating current side end of the converter VSC 2.

The embodiment of the invention also provides a control method of the hybrid direct current third station on-line switching test system, which is used for controlling on-line switching of the converter VSC2 in the hybrid direct current third station on-line switching test system, wherein the converter VSC1 and the converter VSC2 adopt a half-bridge topological structure, the converter LCC and the converter VSC1 are in a steady state running state, the direct current switch AK1, the direct current switch AK2, the direct current switch BK1 and the direct current switch BK2 are all in a combined position, and the direct current switch CK1 and the direct current switch CK2 are all in a split position, and the control method comprises the following steps:

The control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be put into is in a constant voltage control mode;

applying a trigger pulse to the converter VSC2, and setting a target value of the direct current voltage of the converter VSC2 as a direct current voltage value of a voltage control end of the current converter VSC 1;

when the direct current voltage of the converter VSC2 reaches a target value or the direct current voltage of the converter VSC2 and the direct current voltage of the converter VSC1 are in a voltage error range, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch CK1 and the direct current switch CK2 to be closed, so that the converter VSC2 is put into use;

when the direct current switch CK1 and the direct current switch CK2 are at the closed position, the control mode of the converter VSC2 is changed from a constant voltage control mode to a constant active power control mode;

when the power of the converter VSC2 is changed to the power target value of the converter VSC2 according to the preset change rate, the on-line input success of the converter VSC2 is determined.

The embodiment of the invention also provides a control method of the hybrid direct current third station on-line switching test system, which is used for controlling the on-line switching of the converter VSC2 in the hybrid direct current third station on-line switching test system, wherein the converter VSC1 and the converter VSC2 adopt a half-bridge topological structure, the converter LCC, the converter VSC1 and the converter VSC2 are in a steady state running state, and the direct current switch AK1, the direct current switch AK2, the direct current switch BK1, the direct current switch BK2, the direct current switch CK1 and the direct current switch CK2 are all in a combined position, and the control method comprises the following steps:

The control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be withdrawn is in a constant active power control mode;

when the power of the converter VSC2 is reduced to zero and the direct current of the converter VSC2 is zero, a direct current switch-off command is issued, and the direct current switch-off command is used for controlling the direct current switch CK1 and the direct current switch CK2 to switch off so that the converter VSC2 exits;

and switching the control mode of the converter VSC2 from the active power control mode to the voltage control mode, and determining that the converter VSC2 exits successfully on line.

The embodiment of the invention also provides a control method of the hybrid direct current third station on-line switching test system, which is used for controlling on-line switching of the converter VSC2 in the hybrid direct current third station on-line switching test system, wherein the converter VSC1 and the converter VSC2 adopt a full-bridge topology or a full-half-bridge hybrid topology structure, the converter LCC and the converter VSC1 are in a steady state running state, the direct current switch AK1, the direct current switch AK2, the direct current switch BK1 and the direct current switch BK2 are all in a closed position, and the direct current switch CK1 and the direct current switch CK2 are all in a split position, and the control method comprises the following steps:

the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be put into is in a constant direct current voltage control mode;

Issuing a forced phase shifting command to the LCC of the converter so that the triggering angle of the LCC of the converter is forced to be shifted to 160 degrees;

applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a zero voltage and zero current mode input command, and detecting whether the direct current voltage and the direct current reach zero;

when the direct voltage of the hybrid direct current third station on-line switching test system is zero and the direct current is zero, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch CK1 and the direct current switch CK2 to be closed so that the converter VSC2 is put into operation;

when the direct current switch CK1 and the direct current switch CK2 are at the closed position, the control mode of the converter VSC2 is changed from a constant voltage control mode to a constant active power control mode, and the power target value of the converter VSC2 is set to be zero;

applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a command of exiting a zero voltage and zero current mode, controlling the direct current voltage of the converter VSC1 to rise to the rated direct current voltage according to a preset rate, and controlling the converter LCC to cancel forced phase shifting, and controlling the direct current of the converter LCC to rise;

when the converter VSC2 power changes to the power target value, it is determined that the converter VSC2 is successfully put in online.

The embodiment of the invention also provides a control method of the hybrid direct current third station on-line switching test system, which is used for controlling on-line switching of the converter VSC2 in the hybrid direct current third station on-line switching test system, wherein the converter VSC1 and the converter VSC2 adopt a full-bridge topology or a full-half-bridge hybrid topology structure, the converter LCC, the converter VSC1 and the converter VSC2 are in a steady state running state, and the direct current switch AK1, the direct current switch AK2, the direct current switch BK1, the direct current switch BK2, the direct current switch CK1 and the direct current switch CK2 are all in a combined position, and the control method comprises the following steps:

issuing a forced phase shifting command to the converter LCC so that the triggering angle of the converter LCC is forced to be shifted to 160 degrees;

when the direct voltage of the mixed direct current third station on-line switching test system is zero and the direct current is zero, a direct current switch-off command is issued, and the direct current switch-off command is used for controlling the direct current switch CK1 and the direct current switch CK2 to switch off so that the converter VSC2 exits;

When the direct current switch CK1 and the direct current switch CK2 are at the opening position and the direct current of the converter VSC2 is zero, the converter VSC2 is controlled to be switched from a power control mode to a direct current voltage control mode;

when the direct current of the converter LCC rises to a preset value, the converter LCC and the converter VSC1 are determined to be in a stable running state, and the converter VSC2 is determined to be successfully withdrawn online.

The embodiment of the invention also provides a control method of the hybrid direct current third station on-line switching test system, which is used for controlling on-line switching of the converter LCC in the hybrid direct current third station on-line switching test system, wherein the converter VSC1 and the converter VSC2 adopt a half-bridge topological structure, the converter VSC1 and the converter VSC2 are in a steady state running state, the direct current switch BK1, the direct current switch BK2, the direct current switch CK1 and the direct current switch CK2 are all in a combined position, and the direct current switch AK1 and the direct current switch AK2 are all in a split position, and the control method comprises the following steps:

The method comprises the steps of controlling a converter VSC1 to be in a constant direct current voltage control mode, controlling a converter VSC2 to be in a constant active power control mode, and enabling a to-be-put converter LCC to work in an idle load pressurization control mode, wherein a target value of the direct current voltage of the converter LCC is the direct current voltage of the converter VSC 1;

when the direct current voltage of the converter LCC rises to a target value, a direct current switch closing command is issued, wherein the direct current switch closing command is used for controlling the direct current switch AK1 and the direct current switch AK2 to be closed so that the converter LCC is put into operation;

when the direct current switch AK1 and the direct current switch AK2 are at the closed position, the converter LCC exits from the no-load pressurization control mode and is converted into a constant direct current control mode or a constant direct current power control mode;

when the direct current of the converter LCC rises to a preset current value according to a preset change rate, the on-line input success of the converter LCC is determined.

The embodiment of the invention also provides a control method of the hybrid direct current third station on-line switching test system, which is used for controlling on-line switching of the converter LCC in the hybrid direct current third station on-line switching test system, wherein the converter VSC1 and the converter VSC2 adopt a full-bridge topology or a full-half-bridge hybrid topology, the converter VSC1 and the converter VSC2 are in a steady state running state, the direct current switch BK1, the direct current switch BK2, the direct current switch CK1 and the direct current switch CK2 are all in a closed position, and the direct current switch AK1 and the direct current switch AK2 are all in a split position, and the control method comprises the following steps:

The method comprises the steps of controlling a converter VSC1 to be in a constant direct current voltage control mode, controlling a converter VSC2 to be in a constant active power control mode, and controlling a to-be-input converter LCC to be in a constant current control mode or a constant direct current power control mode;

when the direct current voltage of the converter VSC1 is reduced to zero and the power of the converter VSC2 is reduced to zero, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch AK1 and the direct current switch AK2 to be closed, so that the converter LCC is put into operation;

when the direct current switch AK1 and the direct current switch AK2 are in a closed position, trigger pulse is applied to the converter VSC1 or the converter VSC2, the converter VSC1 and the converter VSC2 are controlled to issue a command of exiting a zero-voltage zero-current mode, and the direct current voltage of the converter VSC1 is controlled to rise to a rated direct current voltage;

when the direct current voltage of the converter VSC1 rises to the rated direct current voltage, controlling the converter LCC to be converted into a constant direct current control mode or a constant direct current power control mode, and unlocking the converter LCC;

The embodiment of the invention also provides a control method of the hybrid direct current third station on-line switching test system, which is used for controlling the on-line switching of the converter LCC in the hybrid direct current third station on-line switching test system, wherein the converter LCC, the converter VSC1 and the converter VSC2 are in a steady state running state, and the direct current switch BK1, the direct current switch BK2, the direct current switch CK1, the direct current switch CK2, the direct current switch AK1 and the direct current switch AK2 are all in a combined position, and comprises the following steps:

the method comprises the steps of controlling a converter VSC2 to be in a fixed active power control mode, controlling a converter VSC1 to be in a fixed direct current voltage control mode and controlling a converter LCC to be withdrawn to be in a fixed current control mode;

controlling the direct current of the LCC of the converter to drop to a preset current value, wherein the preset current value is a preset percentage of rated current, and issuing a LCC forced phase shifting command of the converter to forcedly shift the triggering angle to 160 ℃;

when the direct current of the converter LCC is reduced to zero, a direct current switch-off command is issued, wherein the direct current switch-off command is used for controlling the direct current switch AK1 and the direct current switch AK2 to switch off so that the converter LCC exits;

When the direct current switch AK1 and the direct current switch AK2 are in the split state, the converters VSC1 and VSC2 are in a steady state operation state, and the successful online exit of the converter LCC is determined.

In the embodiment of the invention, the third station on-line switching test system of the hybrid direct current system comprises: the system comprises a transformer T1, a transformer T2, a transformer T3, a converter LCC, a converter VSC1 and a converter VSC2, wherein a first end of the transformer T3 is connected with an alternating current power grid S, and a second end of the transformer T3 is connected with an alternating current side end of the converter VSC 1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected to the second end of the transformer T3, and the second end of the transformer T2 is connected to the ac side end of the converter VSC 2. Compared with the technical scheme in the prior art, the embodiment of the invention has a complete test system loop, and can realize the online input and the online exit of the third station in the hybrid direct current system, and can be used for verifying the control and the protection strategy of the online input and the exit of the third station in the hybrid direct current transmission project and verifying the feasibility of the online input and the exit technology of the third station in the actual project.

Drawings

In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art. In the drawings:

FIG. 1 is a schematic diagram of an online switching test system of a hybrid DC third station according to an embodiment of the present invention;

fig. 2 is a flowchart of an on-line input test mode of the converter VSC1 and the converter VSC2 adopting a half-bridge topology structure when the third station is the converter VSC2 in the embodiment of the present invention;

fig. 3 is a flowchart of an online exit test method of the present invention when the third station is the converter VSC2 and the converter VSC1 and the converter VSC2 adopt a half-bridge topology structure;

fig. 4 is a flowchart of an on-line input test method provided in the embodiment of the present invention when the third station is a converter VSC2 and the converter VSC1 and the converter VSC2 adopt a full-bridge topology or a full-half-bridge hybrid topology;

Fig. 5 is a flowchart of an online exit test method provided in the embodiment of the present invention when the third station is a converter VSC2 and the converter VSC1 and the converter VSC2 adopt a full-bridge topology or a full-half-bridge hybrid topology;

fig. 6 is a flowchart of an on-line input test method of the converter VSC1 and the converter VSC2 adopting a half-bridge topology structure when the third station is the converter LCC in the embodiment of the present invention;

fig. 7 is a flowchart of an on-line input test method of using a full-bridge topology or a full-half-bridge hybrid topology for the converter VSC1 and the converter VSC2 when the third station is the converter LCC in the embodiment of the present invention;

fig. 8 is a flowchart of an online exit test method for the LCC converter when the third station is provided in the embodiment of the present invention;

fig. 9 is a flowchart of an online input test mode of the converter VSC2 when the third station is provided in the embodiment of the present invention;

fig. 10 is a field wave-recording diagram of an on-line input of a converter VSC2 provided in an embodiment of the present invention;

fig. 11 is a flowchart of an online exit test mode of the converter VSC2 when the third station is provided in the embodiment of the present invention;

fig. 12 is a field wave recording diagram of an online exit of the converter VSC2 according to an embodiment of the present invention.

Detailed Description

For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the embodiments of the present invention will be described in further detail with reference to the accompanying drawings. The exemplary embodiments of the present invention and their descriptions herein are for the purpose of explaining the present invention, but are not to be construed as limiting the invention.

The embodiment of the invention provides a hybrid direct current third station online switching test system, and fig. 1 is a schematic diagram of the hybrid direct current third station online switching test system provided in the embodiment of the invention, as shown in fig. 1, the hybrid direct current third station online switching test system comprises: a transformer T1, a transformer T2, a transformer T3, a converter LCC, a converter VSC1 and a converter VSC2; wherein, the positive and negative lines of the direct current side of the converter LCC are respectively provided with a direct current switch AK1 and a direct current switch AK2; a direct current switch BK1 and a direct current switch BK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 1; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC 1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating current side end of the converter VSC 2.

In one embodiment, the hybrid dc third station on-line pitch-and-roll test system further comprises: a ground resistor R1 and a ground resistor R2; the direct-current side line of the converter LCC, the direct-current side line of the converter VSC1 and the direct-current side line of the converter VSC2 are grounded through a resistor R1 and a resistor R2.

In one embodiment, the converter LCC may employ a 12-pulse thyristor valve bank, and the converters VSC1, VSC2 may employ a modular multilevel topology; the converter VSC1 and the converter VSC2 may further comprise power modules employing any one of the following topologies: half-bridge topology power module, full-half-bridge hybrid topology power module.

In one embodiment, the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, and the dc switch CK2 may be a high-speed dc switch HSS or a single-phase ac circuit breaker.

Fig. 2 is a flowchart of an on-line input test mode of the converter VSC2 and the converters VSC1 and VSC2 adopting a half-bridge topology structure in the embodiment of the present invention, wherein the converters LCC and VSC1 are in a steady state operation state, the dc switch AK1, the dc switch AK2, the dc switch BK1 and the dc switch BK2 are all in a closed position, and the dc switch CK1 and the dc switch CK2 are all in an open position, as shown in fig. 2, the method includes:

S201: the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be put into is in a constant voltage control mode;

s202: applying a trigger pulse to the converter VSC2, and setting a target value of the direct current voltage of the converter VSC2 as a direct current voltage value of a voltage control end of the current converter VSC 1;

s203: when the direct current voltage of the converter VSC2 reaches a target value or the direct current voltage of the converter VSC2 and the direct current voltage of the converter VSC1 are in a voltage error range, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch CK1 and the direct current switch CK2 to be closed, so that the converter VSC2 is put into use;

s204: when the direct current switch CK1 and the direct current switch CK2 are at the closed position, the control mode of the converter VSC2 is changed from a constant voltage control mode to a constant active power control mode;

s205: when the power of the converter VSC2 is changed to the power target value of the converter VSC2 according to the preset change rate, the on-line input success of the converter VSC2 is determined.

Wherein, the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be put into is in a constant voltage control mode; applying a trigger pulse to the converter VSC2, and setting a target value of the direct current voltage of the converter VSC2 as a direct current voltage value of a voltage control end of the current converter VSC 1; when the direct current voltage of the converter VSC2 reaches a target value or the direct current voltage of the converter VSC2 and the direct current voltage of the converter VSC1 are in a voltage error range, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch CK1 and the direct current switch CK2 to be closed, so that the converter VSC2 is put into use; when the direct current switch CK1 and the direct current switch CK2 are at the closed position, the control mode of the converter VSC2 is changed from a constant voltage control mode to a constant active power control mode; when the power of the converter VSC2 is changed to the power target value of the converter VSC2 according to the preset change rate, the on-line input success of the converter VSC2 is determined. It should be noted that the test procedure should be repeated from the first step until the test is successful, when a problem or trip occurs at any stage of the test procedure.

Here it is explained that the principle of the control method is the same when the third station is a converter VSC 1.

Fig. 3 is a flowchart of an online exit test mode of the converter VSC1 and the converter VSC2 with a half-bridge topology structure when the third station is the converter VSC2, wherein the converter LCC, the converter VSC1 and the converter VSC2 are in a steady state operation state; the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, and the dc switch CK2 are all in the closed position, as shown in fig. 3, and the method includes:

s301: the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be withdrawn is in a constant active power control mode;

s302: when the power of the converter VSC2 is reduced to zero and the direct current of the converter VSC2 is zero, a direct current switch-off command is issued, and the direct current switch-off command is used for controlling the direct current switch CK1 and the direct current switch CK2 to switch off so that the converter VSC2 exits;

s303: and switching the control mode of the converter VSC2 from the active power control mode to the voltage control mode, and determining that the converter VSC2 exits successfully on line.

Wherein, the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be withdrawn is in a constant active power control mode; when the power of the converter VSC2 is reduced to zero and the direct current of the converter VSC2 is zero, a direct current switch-off command is issued, and the direct current switch-off command is used for controlling the direct current switch CK1 and the direct current switch CK2 to switch off so that the converter VSC2 exits; and switching the control mode of the converter VSC2 from the active power control mode to the voltage control mode, and determining that the converter VSC2 exits successfully on line. It should be noted that, if the initial state of the operation of the converter VSC2 is in the voltage control operation mode, the control mode of the converter VSC2 needs to be converted, and after the converter VSC2 is converted to the direct current control power mode or the direct current control mode, the above process is repeated, and the system trips when a problem occurs at any stage of the above test process, the above test process should be repeatedly executed from the first step until the test is successful.

Fig. 4 is a flowchart of an on-line input test method for a full-bridge topology or a full-half-bridge hybrid topology when the third station is the converter VSC2 and the converters VSC1 and VSC2 are in the third station, wherein the converters LCC and VSC1 are in a steady state operation state, the dc switches AK1, AK2, BK1 and BK2 are all in a closed position, and the dc switches CK1 and CK2 are all in a split position, as shown in fig. 4, the method includes:

S401: the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be put into is in a constant direct current voltage control mode;

s402: issuing a forced phase shifting command to the LCC of the converter so that the triggering angle of the LCC of the converter is forced to be shifted to 160 degrees;

s403: applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a zero voltage and zero current mode input command, and detecting whether the direct current voltage and the direct current reach zero;

s404: when the direct voltage of the hybrid direct current third station on-line switching test system is zero and the direct current is zero, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch CK1 and the direct current switch CK2 to be closed so that the converter VSC2 is put into operation;

s405: when the direct current switch CK1 and the direct current switch CK2 are at the closed position, the control mode of the converter VSC2 is changed from a constant voltage control mode to a constant active power control mode, and the power target value of the converter VSC2 is set to be zero;

s406: applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a command of exiting a zero voltage and zero current mode, controlling the direct current voltage of the converter VSC1 to rise to the rated direct current voltage according to a preset rate, and controlling the converter LCC to cancel forced phase shifting, and controlling the direct current of the converter LCC to rise;

S407: when the converter VSC2 power changes to the power target value, it is determined that the converter VSC2 is successfully put in online.

Wherein, the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct voltage control mode, and the converter VSC2 to be put into is in a constant direct voltage control mode; issuing a forced phase shifting command to the LCC of the converter so that the triggering angle of the LCC of the converter is forced to be shifted to 160 degrees; applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a zero voltage and zero current mode input command, and detecting whether the direct current voltage and the direct current reach zero; when the direct voltage of the hybrid direct current third station on-line switching test system is zero and the direct current is zero, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch CK1 and the direct current switch CK2 to be closed so that the converter VSC2 is put into operation; when the direct current switch CK1 and the direct current switch CK2 are at the closed position, the control mode of the converter VSC2 is changed from a constant voltage control mode to a constant active power control mode, and the power target value of the converter VSC2 is set to be zero; applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a command of exiting a zero voltage and zero current mode, controlling the direct current voltage of the converter VSC1 to rise to the rated direct current voltage according to a preset rate, and controlling the converter LCC to cancel forced phase shifting, and controlling the direct current of the converter LCC to rise; when the converter VSC2 power changes to the power target value, it is determined that the converter VSC2 is successfully put in online. It should be noted that, when a problem occurs in any stage of the test procedure, the system trips, and the test procedure should be repeatedly executed from the first step until the test is successful.

Fig. 5 is a flowchart of an online exit test method of a full-bridge topology or a full-half-bridge hybrid topology when the third station is the converter VSC2 and the converters VSC1 and VSC2 are in a steady state operation state, wherein the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1 and the dc switch CK2 are all in a closed position, as shown in fig. 5, the method includes:

s501: the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be withdrawn is in a constant active power control mode;

s502: issuing a forced phase shifting command to the converter LCC so that the triggering angle of the converter LCC is forced to be shifted to 160 degrees;

s503: applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a zero voltage and zero current mode input command, and detecting whether the direct current voltage and the direct current reach zero;

s504: when the direct voltage of the mixed direct current third station on-line switching test system is zero and the direct current is zero, a direct current switch-off command is issued, and the direct current switch-off command is used for controlling the direct current switch CK1 and the direct current switch CK2 to switch off so that the converter VSC2 exits;

S505: when the direct current switch CK1 and the direct current switch CK2 are at the opening position and the direct current of the converter VSC2 is zero, the converter VSC2 is controlled to be switched from a power control mode to a direct current voltage control mode;

s506: applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a command of exiting a zero voltage and zero current mode, controlling the direct current voltage of the converter VSC1 to rise to the rated direct current voltage according to a preset rate, and controlling the converter LCC to cancel forced phase shifting, and controlling the direct current of the converter LCC to rise;

s507: when the direct current of the converter LCC rises to a preset value, the converter LCC and the converter VSC1 are determined to be in a stable running state, and the converter VSC2 is determined to be successfully withdrawn online.

Wherein, the control converter LCC is in a constant current control mode, the converter VSC1 is in a constant direct current voltage control mode, and the converter VSC2 to be withdrawn is in a constant active power control mode; issuing a forced phase shifting command to the converter LCC so that the triggering angle of the converter LCC is forced to be shifted to 160 degrees; applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a zero voltage and zero current mode input command, and detecting whether the direct current voltage and the direct current reach zero; when the direct voltage of the mixed direct current third station on-line switching test system is zero and the direct current is zero, a direct current switch-off command is issued, and the direct current switch-off command is used for controlling the direct current switch CK1 and the direct current switch CK2 to switch off so that the converter VSC2 exits; when the direct current switch CK1 and the direct current switch CK2 are at the opening position and the direct current of the converter VSC2 is zero, the converter VSC2 is controlled to be switched from a power control mode to a direct current voltage control mode; applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a command of exiting a zero voltage and zero current mode, controlling the direct current voltage of the converter VSC1 to rise to the rated direct current voltage according to a preset rate, and controlling the converter LCC to cancel forced phase shifting, and controlling the direct current of the converter LCC to rise; when the direct current of the converter LCC rises to a preset value, the converter LCC and the converter VSC1 are determined to be in a stable running state, and the converter VSC2 is determined to be successfully withdrawn online. It should be noted that, if the initial system operation state is that the converter VSC2 is in the voltage control operation mode, the control mode conversion needs to be performed on the converter VSC2, and after the converter VSC2 is converted into the direct current control power mode or the direct current control mode, the above process is repeated, and the system trip occurs at any stage of the above test process, the above test process should be repeatedly performed from the first step until the test is successful.

Fig. 6 is a flowchart of an on-line input test mode of the converter VSC1 and the converter VSC2 with a half-bridge topology structure when the third station is the converter LCC, wherein the converter VSC1 and the converter VSC2 are in a steady state operation state, the dc switch BK1, the dc switch BK2, the dc switch CK1 and the dc switch CK2 are all in a closed position, and the dc switch AK1 and the dc switch AK2 are all in an open position, as shown in fig. 6, the method includes:

s601: the method comprises the steps of controlling a converter VSC1 to be in a constant direct current voltage control mode, controlling a converter VSC2 to be in a constant active power control mode, and enabling a to-be-put converter LCC to work in an idle load pressurization control mode, wherein a target value of the direct current voltage of the converter LCC is the direct current voltage of the converter VSC 1;

s602: when the direct current voltage of the converter LCC rises to a target value, a direct current switch closing command is issued, wherein the direct current switch closing command is used for controlling the direct current switch AK1 and the direct current switch AK2 to be closed so that the converter LCC is put into operation;

s603: when the direct current switch AK1 and the direct current switch AK2 are at the closed position, the converter LCC exits from the no-load pressurization control mode and is converted into a constant direct current control mode or a constant direct current power control mode;

S604: when the direct current of the converter LCC rises to a preset current value according to a preset change rate, the on-line input success of the converter LCC is determined.

The method comprises the steps of controlling a converter VSC1 to be in a constant direct current voltage control mode, controlling a converter VSC2 to be in a constant active power control mode, and enabling a to-be-put converter LCC to work in an idle load pressurization control mode, wherein the target value of the direct current voltage of the converter LCC is the direct current voltage of the converter VSC 1; when the direct current voltage of the converter LCC rises to a target value, a direct current switch closing command is issued, wherein the direct current switch closing command is used for controlling the direct current switch AK1 and the direct current switch AK2 to be closed so that the converter LCC is put into operation; when the direct current switch AK1 and the direct current switch AK2 are at the closed position, the converter LCC exits from the no-load pressurization control mode and is converted into a constant direct current control mode or a constant direct current power control mode; when the direct current of the converter LCC rises to a preset current value according to a preset change rate, the on-line input success of the converter LCC is determined. It should be noted that, when a problem occurs in any stage of the test procedure, the system trips, and the test procedure should be repeatedly executed from the first step until the test is successful.

Fig. 7 is a flowchart of an on-line input test method for a full-bridge topology or a full-half-bridge hybrid topology when a third station is a converter LCC and the converter VSC1 and the converter VSC2 are in a full-bridge or full-half-bridge hybrid topology, wherein the converter VSC1 and the converter VSC2 are in a steady state operation state, the dc switch BK1, the dc switch BK2, the dc switch CK1 and the dc switch CK2 are all in a closed position, and the dc switch AK1 and the dc switch AK2 are all in a split position, as shown in fig. 7, the method includes:

s701: the method comprises the steps of controlling a converter VSC1 to be in a constant direct current voltage control mode, controlling a converter VSC2 to be in a constant active power control mode, and controlling a to-be-input converter LCC to be in a constant current control mode or a constant direct current power control mode;

s702: applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a zero voltage and zero current mode input command, and detecting whether the direct current voltage and the direct current reach zero;

s703: when the direct current voltage of the converter VSC1 is reduced to zero and the power of the converter VSC2 is reduced to zero, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch AK1 and the direct current switch AK2 to be closed, so that the converter LCC is put into operation;

S704: when the direct current switch AK1 and the direct current switch AK2 are in a closed position, trigger pulse is applied to the converter VSC1 or the converter VSC2, the converter VSC1 and the converter VSC2 are controlled to issue a command of exiting a zero-voltage zero-current mode, and the direct current voltage of the converter VSC1 is controlled to rise to a rated direct current voltage;

s705: when the direct current voltage of the converter VSC1 rises to the rated direct current voltage, controlling the converter LCC to be converted into a constant direct current control mode or a constant direct current power control mode, and unlocking the converter LCC;

s706: when the direct current of the converter LCC rises to a preset current value according to a preset change rate, the on-line input success of the converter LCC is determined.

Wherein, the control converter VSC1 is in a constant direct current voltage control mode, the converter to be put into VSC2 is in a constant active power control mode, the converter to be put into LCC is in a constant current control mode or a constant direct current power control mode; applying trigger pulse to the converter VSC1 or the converter VSC2, controlling the converter VSC1 and the converter VSC2 to issue a zero voltage and zero current mode input command, and detecting whether the direct current voltage and the direct current reach zero; when the direct current voltage of the converter VSC1 is reduced to zero and the power of the converter VSC2 is reduced to zero, a direct current switch closing command is issued, and the direct current switch closing command is used for controlling the direct current switch AK1 and the direct current switch AK2 to be closed, so that the converter LCC is put into operation; when the direct current switch AK1 and the direct current switch AK2 are in a closed position, trigger pulse is applied to the converter VSC1 or the converter VSC2, the converter VSC1 and the converter VSC2 are controlled to issue a command of exiting a zero-voltage zero-current mode, and the direct current voltage of the converter VSC1 is controlled to rise to a rated direct current voltage; when the direct current voltage of the converter VSC1 rises to the rated direct current voltage, controlling the converter LCC to be converted into a constant direct current control mode or a constant direct current power control mode, and unlocking the converter LCC; when the direct current of the converter LCC rises to a preset current value according to a preset change rate, the on-line input success of the converter LCC is determined. It should be noted that, when a problem occurs in any stage of the test procedure, the system trips, and the test procedure should be repeatedly executed from the first step until the test is successful.

Fig. 8 is a flowchart of an online exit test manner of a third station being a converter LCC, where the converter LCC, the converter VSC1, and the converter VSC2 are in a steady state operation state, and the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2, the dc switch AK1, and the dc switch AK2 are all in a closed position, as shown in fig. 8, and the method includes:

s801: the method comprises the steps of controlling a converter VSC2 to be in a fixed active power control mode, controlling a converter VSC1 to be in a fixed direct current voltage control mode and controlling a converter LCC to be withdrawn to be in a fixed current control mode;

s802: controlling the direct current of the LCC of the converter to drop to a preset current value, wherein the preset current value is a preset percentage of rated current, and issuing a LCC forced phase shifting command of the converter to forcedly shift the triggering angle to 160 ℃;

s803: when the direct current of the converter LCC is reduced to zero, a direct current switch-off command is issued, wherein the direct current switch-off command is used for controlling the direct current switch AK1 and the direct current switch AK2 to switch off so that the converter LCC exits;

s804: when the direct current switch AK1 and the direct current switch AK2 are in the split state, the converters VSC1 and VSC2 are in a steady state operation state, and the successful online exit of the converter LCC is determined.

Wherein, the control converter VSC2 is in a fixed active power control mode, the converter VSC1 is in a fixed direct current voltage control mode, and the LCC of the converter to be withdrawn is in a fixed current control mode; controlling the direct current of the LCC of the converter to drop to a preset current value, wherein the preset current value is a preset percentage of rated current, and issuing a LCC forced phase shifting command of the converter to forcedly shift the triggering angle to 160 ℃; when the direct current of the converter LCC is reduced to zero, a direct current switch-off command is issued, wherein the direct current switch-off command is used for controlling the direct current switch AK1 and the direct current switch AK2 to switch off so that the converter LCC exits; when the direct current switch AK1 and the direct current switch AK2 are in the split state, the converters VSC1 and VSC2 are in a steady state operation state, and the successful online exit of the converter LCC is determined. It should be noted that the method for online exiting the test of the LCC of the converter is not affected by the manner adopted by the VSC2 power module of the converter, and the system trips when a problem occurs at any stage of the test process, the test process should be repeatedly executed from the first step until the test is successful.

Fig. 9 is a flow chart of an online input test mode when the third station is a converter VSC2, wherein the converter LCC adopts a 12-pulse thyristor valve group, the dc voltage is ±10.5kV, the rated power is 21MW, and the ac side is connected with a transformer with rated capacity of 24.4 MW; the rated power of the converter VSC1 and the rated power of the converter VSC2 are 66MW, a full-half-bridge hybrid topological structure is adopted, the proportion of full-half-bridge modules is 10:2, each bridge arm contains two modules for redundancy, and the alternating-current side of the converter VSC2 is connected with a transformer with rated capacity of 75 MVA; the dc pole line switches AK1, AK2, BK1, BK2 of the converter LCC and VSC1 are in the combined position, and the dc pole line switches CK1, CK2 of the converter VSC2 are in the split position, as shown in fig. 9, the method includes:

S901: the constant voltage unlocking converter VSC1 and the constant current unlocking converter LCC enable the converter LCC and the converter VSC1 to be in a steady-state operation stage, and direct-current voltage is controlled to be a rated value;

s902: converter VSC2 that fixed voltage unblock needs to put into;

s903: controlling the converter LCC to issue a forced phase shift command to enable the triggering angle to be forced to be shifted to 160 degrees, and simultaneously controlling the converters VSC1 and VSC2 to issue a zero-voltage zero-flow control mode;

s904: when the current direct current voltage is zero and the direct current is zero, issuing a closing command of the current converter VSC2 polar line switches CK1 and CK2 which are needed to be put into, and confirming that the current converter VSC2 polar line switches CK1 and CK2 are in a closing position;

s905: the control mode of the current converter VSC2 needing to be put into is controlled to be changed from a constant voltage control mode to a constant power control mode, and the converter VSC2 is controlled to exit a zero voltage and zero current mode;

s906: the LCC of the converter is controlled to cancel the forced phase shift, and when the direct current voltage is gradually recovered to the rated value, the third station is put into operation successfully.

It should be noted that in this embodiment, fig. 10 is a field wave recording diagram of an on-line input of a converter VSC2 provided in the embodiment of the present invention, as shown in fig. 10, in which S1 is a converter LCC, S2 is a converter VSC1, S3 is a converter VSC2, and 20B03, 20B04 are single-phase ac circuit breakers of positive and negative lines.

Fig. 11 is a flow chart of an online exit test mode of the converter VSC2 when the third station is the converter, wherein the converter LCC adopts a 12-pulse thyristor valve group, the dc voltage is ±10.5kV, the rated power is 21MW, and the ac side is connected with a transformer with a rated capacity of 24.4 MW; the rated power of the converter VSC1 and the rated power of the converter VSC2 are 66MW, a full-half-bridge hybrid topological structure is adopted, the proportion of full-half-bridge modules is 10:2, each bridge arm contains two modules for redundancy, the alternating current side of the converter VSC2 is connected with a transformer with rated capacity of 75MVA, and the switches AK1, AK2, BK1, BK2, CK1 and CK2 at the two ends of a direct current line of the converter LCC, the converter VSC1 and the converter VSC2 are in a combined position, as shown in FIG. 11, the method comprises the following steps:

s1101: constant-voltage unlocking converter VSC1, constant-current unlocking converter LCC, and constant-power unlocking of converter VSC2 to be withdrawn.

S1102: when the converter LCC, the converter VSC1 and the converter VSC2 are in a steady state operation state, issuing a converter LCC forced phase shifting command to enable the triggering angle of the converter LCC to be forced to be shifted to 160 degrees, and putting the converter VSC1 and the converter VSC2 into a zero-voltage zero-current mode;

s1103: when the system direct current voltage is zero and the direct current is zero, issuing a switching-off command of the line switches CK1 and CK2 of the converter VSC 2;

S1104: when the line switches CK1 and CK2 of the converter VSC2 are in the split position, the converter VSC2 is switched from a power control mode to a voltage control mode;

s1105: and controlling the LCC of the converter to cancel the forced phase shifting, controlling the VSC1 and VSC2 of the converter to exit the zero-voltage zero-current mode, and when the direct-current voltage is recovered, successfully exiting the VSC2 on line.

It should be noted that in this embodiment, fig. 12 is a field wave-recording diagram of on-line exit of the converter VSC2 provided in the embodiment of the present invention, as shown in fig. 12, in which S1 is a converter LCC, S2 is a converter VSC1, S3 is a converter VSC2, and 20B03, 20B04 are single-phase ac circuit breakers of positive and negative lines.

It will be appreciated by those skilled in the art that embodiments of the present invention may be provided as a method, system, or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present invention may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.

The present invention is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.

These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.

The foregoing description of the embodiments has been provided for the purpose of illustrating the general principles of the invention, and is not meant to limit the scope of the invention, but to limit the invention to the particular embodiments, and any modifications, equivalents, improvements, etc. that fall within the spirit and principles of the invention are intended to be included within the scope of the invention.

Claims

1. The control method of the hybrid direct current third station online switching test system is characterized by being used for controlling online switching of the converter VSC2 in the hybrid direct current third station online switching test system, wherein the hybrid direct current third station online switching test system further comprises: a transformer T1, a transformer T2, a transformer T3, a converter LCC and a converter VSC1; a direct current switch AK1 and a direct current switch AK2 are respectively arranged at the positive line and the negative line of the direct current side of the converter LCC; a direct current switch BK1 and a direct current switch BK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC1; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating-current side end of the converter VSC 2; the converter VSC1 and the converter VSC2 adopt a half-bridge topology structure, the converter LCC and the converter VSC1 are in a steady state operation state, the dc switch AK1, the dc switch AK2, the dc switch BK1 and the dc switch BK2 are all in a combined position, the dc switch CK1 and the dc switch CK2 are all in a separated position, and the method comprises the following steps:

2. The method of claim 1, wherein the hybrid dc third station online pitch and catch test system further comprises: a resistor R1 and a resistor R2;

the direct current side line of the converter LCC, the direct current side line of the converter VSC1 and the direct current side line of the converter VSC2 are grounded via a resistor R1 and a resistor R2.

3. The method of claim 1, wherein the converter LCC employs a 12-pulse thyristor valve bank and the converters VSC1, VSC2 employ a modular multilevel topology.

4. A method according to claim 3, characterized in that the converter VSC1 and the converter VSC2 comprise power modules with any one of the following topologies: half-bridge topology power module, full-half-bridge hybrid topology power module.

5. The method of claim 1, wherein the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2 are all high-speed dc switches HSS or single-phase ac circuit breakers.

6. The control method of the hybrid direct current third station online switching test system is characterized by being used for controlling online switching of the converter VSC2 in the hybrid direct current third station online switching test system, wherein the hybrid direct current third station online switching test system further comprises: a transformer T1, a transformer T2, a transformer T3, a converter LCC and a converter VSC1; a direct current switch AK1 and a direct current switch AK2 are respectively arranged at the positive line and the negative line of the direct current side of the converter LCC; the direct current side of the converter VSC1 is positive the negative electrode line is respectively provided with a direct current switch BK1 and a direct current switch BK2; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating-current side end of the converter VSC 2; the converter VSC1 and the converter VSC2 adopt half-bridge topology structures, the converter LCC, the converter VSC1 and the converter VSC2 are in a steady state operation state, and the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1 and the dc switch CK2 are all in a closed position, including:

when the power of the converter VSC2 is reduced to zero and the direct current of the converter VSC2 is zero, a direct current switch-off command is issued, and the direct current switch-off command is used for controlling the direct current switch CK1 and the direct current switch CK2 to switch off so that the converter VSC2 is out of use;

7. The method of claim 6, wherein the hybrid dc third station online pitch and catch test system further comprises: a resistor R1 and a resistor R2;

8. The method of claim 6, wherein the converter LCC employs a 12-pulse thyristor valve bank and the converters VSC1, VSC2 employ a modular multilevel topology.

9. The method of claim 8, wherein the converter VSC1 and the converter VSC2 comprise power modules employing any one of the following topologies: half-bridge topology power module, full-half-bridge hybrid topology power module.

10. The method of claim 6, wherein the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2 each employ a high-speed dc switch HSS or a single-phase ac circuit breaker.

11. The control method of the hybrid direct current third station online switching test system is characterized by being used for controlling online switching of the converter VSC2 in the hybrid direct current third station online switching test system, wherein the hybrid direct current third station online switching test system further comprises: a transformer T1, a transformer T2, a transformer T3, a converter LCC and a converter VSC1; a direct current switch AK1 and a direct current switch AK2 are respectively arranged at the positive line and the negative line of the direct current side of the converter LCC; a direct current switch BK1 and a direct current switch BK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC1; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating-current side end of the converter VSC 2; the converter VSC1 and the converter VSC2 adopt full-bridge topology or full-half-bridge hybrid topology, the converter LCC and the converter VSC1 are in a steady state operation state, the dc switch AK1, the dc switch AK2, the dc switch BK1 and the dc switch BK2 are all in a combined position, the dc switch CK1 and the dc switch CK2 are all in a split position, and the method comprises the following steps:

12. The method of claim 11, wherein the hybrid dc third station online pitch and catch test system further comprises: a resistor R1 and a resistor R2;

13. The method of claim 11, wherein the converter LCC employs a 12-pulse thyristor valve bank and the converters VSC1, VSC2 employ a modular multilevel topology.

14. The method according to claim 13, wherein the converter VSC1 and the converter VSC2 comprise power modules employing any one of the following topologies: half-bridge topology power module, full-half-bridge hybrid topology power module.

15. The method of claim 11, wherein the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2 each employ a high-speed dc switch HSS or a single-phase ac circuit breaker.

16. The control method of the hybrid direct current third station online switching test system is characterized by being used for controlling online switching of the converter VSC2 in the hybrid direct current third station online switching test system, wherein the hybrid direct current third station online switching test system further comprises: a transformer T1, a transformer T2, a transformer T3, a converter LCC and a converter VSC1; a direct current switch AK1 and a direct current switch AK2 are respectively arranged at the positive line and the negative line of the direct current side of the converter LCC; a direct current switch BK1 and a direct current switch BK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC1; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating-current side end of the converter VSC 2; the converter VSC1 and the converter VSC2 adopt full-bridge topology or full-half-bridge hybrid topology, the converter LCC, the converter VSC1 and the converter VSC2 are in a steady state operation state, and the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1 and the dc switch CK2 are all in a combined state, including:

17. The method of claim 16, wherein the hybrid dc third station online pitch and catch test system further comprises: a resistor R1 and a resistor R2;

18. The method of claim 16 wherein the converter LCC employs a 12-pulse thyristor valve bank and the converters VSC1, VSC2 employ a modular multilevel topology.

19. The method of claim 18, wherein the converter VSC1 and the converter VSC2 comprise power modules employing any one of the following topologies: half-bridge topology power module, full-half-bridge hybrid topology power module.

20. The method of claim 16, wherein the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2 each employ a high-speed dc switch HSS or a single-phase ac circuit breaker.

21. The control method of the hybrid direct current third station on-line switching test system is characterized by being used for controlling on-line switching of an inverter LCC in the hybrid direct current third station on-line switching test system, wherein the hybrid direct current third station on-line switching test system further comprises: a transformer T1, a transformer T2, a transformer T3, a converter VSC1 and a converter VSC2; a direct current switch AK1 and a direct current switch AK2 are respectively arranged at the positive line and the negative line of the direct current side of the converter LCC; a direct current switch BK1 and a direct current switch BK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 1; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC 1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating-current side end of the converter VSC2; the converter VSC1 and the converter VSC2 adopt a half-bridge topology structure, the converter VSC1 and the converter VSC2 are in a steady state operation state, the direct current switch BK1, the direct current switch BK2, the direct current switch CK1 and the direct current switch CK2 are all in a closed position, the direct current switch AK1 and the direct current switch AK2 are all in a split position, and the method comprises the following steps:

22. The method of claim 21, wherein the hybrid dc third station online pitch and catch test system further comprises: a resistor R1 and a resistor R2;

23. The method of claim 21 wherein the converter LCC employs a 12-pulse thyristor valve bank and the converters VSC1, VSC2 employ a modular multilevel topology.

24. The method of claim 23, wherein the converter VSC1 and the converter VSC2 comprise power modules employing any one of the following topologies: half-bridge topology power module, full-half-bridge hybrid topology power module.

25. The method of claim 21, wherein the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2 each employ a high-speed dc switch HSS or a single-phase ac circuit breaker.

26. The control method of the hybrid direct current third station on-line switching test system is characterized by being used for controlling on-line switching of an inverter LCC in the hybrid direct current third station on-line switching test system, wherein the hybrid direct current third station on-line switching test system further comprises: a transformer T1, a transformer T2, a transformer T3, a converter VSC1 and a converter VSC2; a direct current switch AK1 and a direct current switch AK2 are respectively arranged at the positive line and the negative line of the direct current side of the converter LCC; a direct current switch BK1 and a direct current switch BK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 1; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC 1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating-current side end of the converter VSC2; the converter VSC1 and the converter VSC2 adopt full-bridge topology or full-half-bridge hybrid topology, the converter VSC1 and the converter VSC2 are in a steady state operation state, the direct current switch BK1, the direct current switch BK2, the direct current switch CK1 and the direct current switch CK2 are all in a combined position, the direct current switch AK1 and the direct current switch AK2 are all in a separated position, and the method comprises the following steps:

27. The method of claim 26, wherein the hybrid dc third station online pitch and catch test system further comprises: a resistor R1 and a resistor R2;

28. The method of claim 26 wherein said converter LCC employs a 12-pulse thyristor valve bank and said converters VSC1, VSC2 employ a modular multilevel topology.

29. The method of claim 28, wherein the converter VSC1 and the converter VSC2 comprise power modules employing any one of the following topologies: half-bridge topology power module, full-half-bridge hybrid topology power module.

30. The method of claim 26, wherein the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2 each employ a high-speed dc switch HSS or a single-phase ac circuit breaker.

31. The control method of the hybrid direct current third station on-line switching test system is characterized by being used for controlling on-line switching of the converter LCC in the hybrid direct current third station on-line switching test system, wherein the hybrid direct current third station on-line switching test system further comprises: a transformer T1, a transformer T2, a transformer T3, a converter VSC1 and a converter VSC2; a direct current switch AK1 and a direct current switch AK2 are respectively arranged at the positive line and the negative line of the direct current side of the converter LCC; a direct current switch BK1 and a direct current switch BK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC 1; a direct current switch CK1 and a direct current switch CK2 are respectively arranged at the positive electrode line and the negative electrode line of the direct current side of the converter VSC2; the first end of the transformer T3 is connected with an alternating current power grid S, and the second end of the transformer T3 is connected with the alternating current side end of the converter VSC 1; the first end of the transformer T1 is connected with the second end of the transformer T3, and the second end of the transformer T1 is connected with the alternating current side end of the converter LCC; the first end of the transformer T2 is connected with the second end of the transformer T3, and the second end of the transformer T2 is connected with the alternating-current side end of the converter VSC2; the converter LCC, the converter VSC1, the converter VSC2 are in a steady state operation state, the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2, the dc switch AK1, the dc switch AK2 are all in a closed position, including:

32. The method of claim 31, wherein the hybrid dc third station online pitch and catch test system further comprises: a resistor R1 and a resistor R2;

33. The method of claim 31 wherein said converter LCC employs a 12-pulse thyristor valve bank and said converters VSC1, VSC2 employ a modular multilevel topology.

34. The method of claim 33 wherein the converter VSC1 and the converter VSC2 comprise power modules employing any one of the following topologies: half-bridge topology power module, full-half-bridge hybrid topology power module.

35. The method of claim 31, wherein the dc switch AK1, the dc switch AK2, the dc switch BK1, the dc switch BK2, the dc switch CK1, the dc switch CK2 each employ a high-speed dc switch HSS or a single-phase ac circuit breaker.